(1) Field of the Invention
The present invention relates to devices for testing the profile of ultrasonic beams, and, more specifically, the present invention relates to devices for testing the profile of ultrasonic beams generated by transducers such as those in medical diagnostic instruments.
(2) Description of the Prior Art
An ultrasonic scanner will interchangeably employ transducers or an array of transducers that generate different beam patterns. The transducers typically have different diameters and operate at different frequencies, and therefore generate beams having different patterns. It is important to select a transducer or an array of transducers that generate a beam having a focal zone encompassing the target of interest. For example, a transducer producing a beam having a deep focal zone produces a sharper image of a deep target, for example a target located 12 centimeters from the transducer face, than a transducer producing a beam having a medium depth focal zone.
A device for testing the profile of an ultrasonic beam is disclosed in "Transducer Beam Pattern Test Object", Hafner et al., J. Clin. Ultrasound 8:5-10, February, 1980. This test object contains a row of seventeen stainless steel rods fixed in a plane by four plastic braces. The test object is immersed in a tank containing room temperature distilled water. A transducer is passed over the test object in a direction transverse to the plane in which the rods are fixed and a partial image of the transducer beam profile is generated on B-mode hard copy. The row of rods of the test object appears as a row of horizontal lines on the image screen. The length of the lines is an indicator of the lateral resolution, or beam width, at the depths of the rods. This test object has several disadvantages. Water used to fill the tank must be degassed, for example, by allowing it to stand at room temperature for two days prior to use. When the test object is submerged into the degassed water, formation of air bubbles occurs. Although the number of air bubbles may be minimized by slowly submerging the test object and/or by adding detergent to the water, bubble formation is likely to occur causing scattering of the beam. Moreover, since the velocity of sound in room temperature water is lower than the average velocity of ultrasound in soft tissue (1540 m/sec), the depth measurement is inaccurate. The object is cumbersome to manipulate in a clinical environment, as distinguished from a laboratory environment, because water spillage may occur and the object is bulky and heavy.
Another device for testing beam profiles is disclosed in "An Automated Ultrasound Transducer Beam Profiling System", Goodsitt et. al., Radiology 132:220-222, July 1979. This device includes an aluminum frame assembly that supports a spherical stainless steel target moveable with respect to the transducer in three degrees of movement. Information about the beam profile is obtained by incrementally moving the target and scanning the target at each target position.